In the industrialized world of today, most homes and businesses are connected to telephone networks using twisted pair copper wires, Those copper wires were originally used solely to carry data traffic in the analogue voice band. However, with the advancement of technology, and in particular the advancement of Digital Subscriber Line, DSL, access technology, the transfer of data over the higher frequencies in the twisted pair copper wires was made possible. The greatest advantage of DSL is that it enables data to be exchanged over the twisted pair copper wires at much higher speeds than conventional modems and analogue lines. The speeds at which data is exchanged over DSL now exceed 200 Mbit/s downstream using the current access technology standard very-high-bit-rate digital subscriber line 2 (VDLS2). Despite high transmission rates of today, DSL access technology is still being developed towards even higher transmission rates. A working name used in current standardization efforts for the next generation of time-division duplex DSL-based access technology is “G.fast”.
In more detail, G.fast is the ITU-T internal working name for a future communication standard intended to provide close to gigabit rates on short copper cables, e.g. 100-200 meters of telephony cable.
DSL communication systems support high-speed data links between on the one side a distribution point unit, possibly being part of central office equipment of a communication service provider, operator or network operator, and on the other side one or more residential network terminals serviced by the distribution point unit. In an ADSL or IDSL communication system, an available spectrum is subdivided into a plurality of tones, each of which carries either downstream information from the distribution point unit to a customer premises equipment, or upstream information from the customer premises equipment to the distribution point unit. While the distribution point unit may provide service to a plurality of customer premises equipments, each piece of serviced customer premises equipment is coupled to the distribution point unit via a respective twisted pair of wires. Often, a large number of customer premises equipment connections are bundled together in one cable, That cable is in turn connected to a cabinet managed by an operator, network operator or other service provider.
The quality of the communication channel provided by a respective twisted pair or wires that couple the customer premises equipment to its servicing distribution point unit will have a substantial impact on the transmission capacity and quality that may be achieved between the communicating terminals. The quality of the communications channel between the distribution point equipment and the serviced customer premises equipment depends upon a number of factors. One of these factors is distance, i.e. the distance from the distribution point unit to the customer premises equipment, as signal attenuation increases with increased distance. Another factor is media quality, e.g. type of media, number of connections, etc. Still another factor is interference that may be coupled to the twisted pair of copper wires, often produced by a neighboring connection or by neighboring customer premises equipment that transmit signals in the same frequency band.
Single Ended Line Testing, SELT, is practically a standardized test method for loop qualification, performance predication/estimation and troubleshooting of copper cables. It consists of two kinds of measurements. The first is a noise measurement typically referred to as Quiet Line Noise, QLN, test and the second is echo measurement.
Echo measurements can be implemented in different ways, for instance as time domain reflectometry, TDR, using steps or pulses, or as frequency domain reflectometry, FDR, using continuous narrowband or wideband signals. Conversion between the time domain and the frequency domain is possible using for example various Fast Fourier Transform, FFT, techniques. From echo measurements, one can determine return loss (i.e. S11), input impedance, and echo response.
US patent application 2011/0161027 describes how to make at least two echo measurements with different far-end impedances, for open lines and short-circuited lines, whereby SELT estimation of a transfer function for a line can be improved, SELT is typically performed at the central office or on the DSLAM side of the communication line and thus, the line test is remotely activated by the operator. To keep the cost of operation as low as possible, it is desirable to use methods that that avoid the need, for manual intervention by operators at the far-end side. Also because of cost reasons and the desire to avoid the addition of unnecessary complexity, it cannot be motivated to use of a specific piece of equipment for altering the impedance at the far-end side. A further limitation, which is related to the disclosure of the patent application referred to, is that both open termination and short-circuit termination is needed at the far-end side, while active customer premises equipment typically has an impedance value which is closely matched to the impedance value of the communication line.
One of the shortcomings of SELT with active customer premises equipment is that matched impedance will weaken the echo signal from the CPE. If there are other echo sources, such as splices and bridge-taps, a signal from those other echo sources may even be stronger than the echo signal reflected from the CPE and therefore it will be hiding that particular echo signal. With strong echo reflections and a hidden CPE echo signal, the location of the CPE becomes difficult to determine, which is important for further localization of faults and for loop qualification. Strong echoes are easier to localize, but even with an open-end terminal, i.e. without a CPE terminal, it gets difficult to identify the echo belonging to the far-end side terminal.
The above mentioned shortcomings and problems related to prior art therefore need to be solved.